Drilling operations offshore from a floating vessel require the deployment, use and disconnect of marine drilling riser. The riser is a conduit and containment vessel for the drill string, drilling fluids and cuttings from the well, and for well gas that may need to be diverted in well control operations.
At the sea bottom, the lower end of the riser is connected to a lower flex joint or ball joint which is part of a mechanism called the Lower Marine Riser Package (LMRP), which in turn is connected at the top of a Ram-type blow out preventer (BOP) that is mounted on top of the wellhead connector. The riser assembly is connected at its upper end to the drilling vessel by way of a telescoping or slip joint and an upper flex or ball joint, to a diverter housing which is located below the drill floor of the vessel.
Marine risers are well known. They are made up of multiple sections of large diameter steel tubes (Marine Riser) that are joined with special connectors. The riser usually supports what are called the kill and choke lines, mud booster line, and other ancillary lines that are connected from the drilling vessel to the wellhead connector.
The drilling riser may be equipped with buoyancy modules of a known type which are either filled with syntactic foam or are hollow vessels that can be filled with air to adjust the buoyancy of the riser. The riser is typically tensioned at the top and connected to the drilling vessel by way of a telescoping slip joint. The slip joint permits relative vertical movement of the drilling vessel with respect to the stationary tensioned riser. Horizontal movement of the riser is facilitated by means of the ball or flex joints.
When the drilling riser is deployed and connected, it is affected and stressed by different interacting forces including its own weight, its top tension, the weight of the drilling fluid, the wave and current action in the water, and the horizontal excursions or movements of the drilling vessel. The riser must be designed to withstand all of these forces in a safe and effective way under normal and extreme conditions.
When the drilling vessel is caused to move by wave and current action greater than normal because of bad weather, a larger riser angle relative to the drilling vessel and/or BOP results causing significant tension and bending stresses due to the weight of the riser and drilling fluid contained in it. The buoyancy modules are provided to mitigate these stresses by reducing the effective weight of the riser in water and even create a positive buoyancy condition.
During deployment or recovery, the riser string is run in an open mode through the diverter housing, which means that there is free flooding in the riser for equalizing the pressure both inside and outside of the riser. During this time, the riser string is hanging freely, either from a derrick or from a spider on the rotary table on the drilling rig. Joints are added to the top of the riser by way of special connectors. Movement of the drilling vessel, as well as wave and current action, act on the hanging riser that is being deployed. Because of the attached buoyancy modules, the riser has an effectively low weight, but a relatively large mass.
This imbalance of mass and weight can lead to severe complications during deployment or when hung-off in very deep water, for example, water depths of greater than 5,000, up to and beyond 10,000 feet, in extreme conditions caused by bad weather. The vessel heaves or moves up and down in the water inducing vertical motion at the top of the riser. This up and down movement can result in severe compressive budding and failure.
Attempts to solve these problems in the past include any one or a combination of different methods of maintaining weight in the lower portion of the riser, but at the same time keeping the weight at a minimum in the upper portion of the riser. One attempt was to increase the thickess and therefore the weight of the lower riser tube. Another consideration was to add an artificially heavy weight, such as spent uranium or the like, to the LMRP. Other attempts included reducing buoyancy at the bottom of the riser string while increasing buoyancy at the top of the riser string, or increasing riser tube thickness. Another method used was to equip riser joints with air-can buoyancy at the lower part of the riser which will add to the weight of the lower part of the riser until air is introduced into the buoyancy modules from a vessel-based, high pressure generator system.
All of these systems have proved to be either more costly or time-consuming than desirable. Thus, there is a need in the industry for an inexpensive and easily operable system and method for adjusting the weight of a riser assembly at the lower end during deployment or disconnect conditions.